In a generally employed refrigerating air conditioning system, for example, an air conditioner of heat pump type, an outdoor air temperature and a refrigerant vaporization temperature of the outdoor heat exchanger are detected, and the difference between those detected temperatures at a predetermined time elapsing from start of heating operation is compared with the difference between those temperatures at a predetermined time when the frost is expected to be formed. When the difference derived from the comparison exceeds a predetermined value, the system starts defrosting.
After an elapse of 20 minutes from start of heating operation, the refrigerating air conditioning system such as the air conditioner detects the outdoor air temperature and the refrigerant temperature, and the difference TA between those temperatures is stored. Then the difference TB between the detected temperatures after an elapse of a predetermined time period is calculated. When the difference between the TA and TB exceeds a set value TC, the defrosting is started. Depending on either high or low outdoor air temperature, the temperature difference TA is set to large or small reference value. Based on the reference value, the determination with respect to the frost formation may be made (for example, see Patent Document 1)
The refrigerating air conditioning system such as another air conditioner of heat pump type is provided with a refrigerant temperature sensor disposed between an indoor heat exchanger and a flow path switching valve, and an outdoor air temperature sensor. The system is designed to stop defrosting when the difference between values detected by the respective sensors becomes equal to or larger than a predetermined value.
The refrigerating air conditioning system like the air conditioner as described above is provided with a frost detection unit including a heat exchange temperature sensor for an outdoor heat exchanger and an air flow pressure sensor for detecting the pressure of air flowing through the outdoor heat exchanger. The system is designed to start defrosting when the temperature is equal to or lower than the predetermined temperature value, and the pressure is equal to or higher than the predetermined pressure value (for example, see Patent Document 2).
The refrigerating air conditioning system as another type of air conditioner is provided with an outdoor pipe temperature detection unit for detecting the temperature of the outdoor heat exchanger during heating operation, and an outdoor air temperature detection unit. The system is designed to determine the frost forming state based on the outdoor heat exchanger temperature, the outdoor air temperature, and a period for operating the compressor.
The system is designed to start defrosting when the outdoor heat exchanger temperature is maintained below the line L1 relative to the outdoor air temperature for 20 minutes or longer, and the compressor operating duration elapses 35 minutes (for example, see Patent Document 3).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 57-164245 (pp. 2-3, FIGS. 3-5)    Patent Document 2: Japanese Unexamined Patent Application Publication No. 60-218551 (pp. 2-3, FIG. 1)    Patent Document 3: Japanese Unexamined Patent Application Publication No. 11-23112 (pp. 2-6, FIG. 3)